Growth hormone (GH) is an anabolic anterior pituitary hormone that stimulates cellular proliferation and differentiation through the synergistic action of GH and insulin-like growth factor-1 (IGF-1). In vivo, the biosynthesis and secretion of GH is regulated by the balance of growth hormone releasing hormone (GHRH) and somatostatin (SST). GH secretion is also subject to feedback mechanisms of control at both the hypothalamus and the pituitary. Secretion of GH is critical to normal skeletal growth during childhood, with maximum secretion occurring during puberty. Deficient secretion of GH in children results in short stature, retarded height velocity, and delayed bone maturation.
A subset of the 3% of children with short stature are growth hormone deficient (GHD), with a prevalence of approximately 1 child per 3700 to 4000. Currently the diagnosis of GH deficiency (GHD) is made in children first based on their slow growth and short stature and delay in bone age. The diagnosis is then confirmed by performing a stimulation test of GH secretion. These standard stimulation tests include insulin induced hypoglycemia, infusion of aginine, glucagon administration subcutaneously, or oral administration of levo-dopa or clonidine. Although insulin induced hypoglycemia is considered the most reliable it requires that the child is supervised by a physician for the two hours that the test takes; in addition adverse effects that include two reported deaths have occurred during insulin induced hypoglycemia (REFS). For these reasons insulin induced hypoglycemia is not used by pediatric endocrinologists and the other tests are used. The glucagon test is probably the most reliable but, to further enhance reliability, two tests are usually performed. Recently growth hormone secretagogue receptor (GHS-R) agonists have been used and they have the advantage of being reliable but the peak GH response defining GH deficiency is poorly characterized. It would be beneficial to develop a GHD test that is easier to use, safer, and/or more reliable than the current tests.
Ibutamoren mesylate (MK-0677) was developed at Merck Research Laboratories (Merck) as a specific orally active growth hormone secretagogue. Merck conducted a phase IIb study of children with variable degrees of short stature and growth hormone deficiency (GHD) in 1996-98. They treated the children with either placebo or the growth hormone secretagogue receptor agonist MK-0677 or with rhGH (recombinant hormone growth hormone). MK-0677 mimics the effect of the now recognized natural ligand for the growth hormone secretagogue receptor, which is the hormone ghrelin. The rationale was to determine whether oral therapy with MK0677 would accelerate growth effectively in children with short stature.
In the Phase IIb study, 24 children were treated with 0.8 mg/kg/d MK-0677. These children had a baseline growth rate of 3.4±1.7 cm/y (centimeter/year), which increased to 6.8±2.0 cm/y at 6 months with a significant change in growth rate of 3.4±2.1 cm/y. This can also be expressed as a height velocity standard deviation (SD) (or standard deviations—also SD) at baseline of 0.4±2.1 and 3.5±2.0 at 6 months. In contrast a group of 22 children treated with placebo had a baseline growth rate of 4.2±1.8 cm/y and 4.6±1.4 cm/year at 6 months with a statistically insignificant change in growth rate of 0.4±2.3 cm/y. Change in height velocity SD for chronological age was 0.4±2.1 for placebo and 3.5±2.0 for treatment with MK-0677 after 6 months at a dose of 0.8 mg/kg/d. Twenty of the 22 placebo-treated children were then treated with standard Growth Hormone (GH) treatment (daily subcutaneous injection of rhGH, 0.043 mg/kg/day). These showed an increase in their height velocity SD score for chronological age from 0.3±2.2 at baseline (i.e., 6 months of placebo treatment) to 7.6±5.6. Since the increase in height velocity SD was twice as high for GH than for MK-0677 treatment, the project was discontinued as MK-0677 was deemed less effective and not competitive with the standard GH therapy.
GHD leading to short stature (−2 SD height for chronological age) in children is a disorder found worldwide. Treatment of growth hormone deficient children having short stature lasts typically for many years from diagnosis in childhood to reaching final height. Results obtained from 6 months assessment of treatment in newly-diagnosed children can be widely variable due to the differences in underlying etilology of the GH deficiency, and patterns and rates of catch-up growth on start of treatment. Typically treatment for 1 year or longer is necessary to establish a new growth trajectory on treatment. Thereafter, treatment is often required for 10 years or more, to reach an optimal adult height in these children. Children with GHD are usually treated by daily subcutaneous injections of GH, which can be painful, inconvenient, and cause distress in some, especially younger, children. It would be beneficial in terms of ease of treatment, patient convenience, and long-term adherence to develop non-injection based therapies, e.g., a once-per-day oral treatment, if such therapies could be shown to have similar efficacy to GH in some groups of GHD patients.